Add protection against slowloris vulnerability

This changes the behavior of the `timeout` protocol option to
mean "Time in which the full request line and headers must be
received". The default of 5s should be fine for all normal uses.

This change has no noticeable impact on performance and is thus
enabled by default for everyone. It can be disabled by setting
`timeout` to `infinity` although that is definitely not encouraged.

Inspired by the contribution from @naryl on github.